System and method for characterizing a printing device

ABSTRACT

A method and a system are described for characterizing a printing device, wherein the system includes a forward look up table for obtaining color values as a function of given colorant values for the printing device. The forward look up table includes a plurality of sampling points, wherein at least one of the sampling points is an ink changing point.

[0001] This application claims the priority of US ProvisionalApplication No. 60/373,526 filed Apr. 18, 2002.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of image rendering bymeans of printing devices, particularly multicolor output devices; theinvention especially concerns characterization of these devices.

BACKGROUND OF THE INVENTION AND DEFINITION OF TERMS

[0003] Today, more and more printing systems are developed for thereproduction of color images. Several printing technologies are usedsuch as conventional photography, electrophotography, thermal transfer,dye sublimation and ink jet systems to name a few.

[0004] All these systems can be described as multidimensional colorprinters with n colorants, such as the CMYK (cyan, magenta, yellow andblack) inks of an ink jet system. In this document it is assumed thatthe colorant values range from 0% (no colorant laid down on thereceiving substrate such as paper) to 100% (maximum amount of colorantlaid down on the receiving substrate).

[0005] With colorant space is meant an n-dimensional space with n thenumber of independent variables with which the printer can be addressed.In the case of an offset printing press the dimension of the spacecorresponds to the number of inks of the printer. When CMYK inks areused, the dimension of the colorant space is normally four.

[0006] The colorant gamut is defined by the possible combinations of thecolorant values, normally going from 0% to 100%. If there are nocolorant limitations, the colorant gamut is a n-dimensional cube.

[0007] With color space is meant a space that represents a number ofquantities of an object that characterize its color. In most practicalsituations, colors will be represented in a 3-dimensional space such asthe CIE XYZ space. However, also other characteristics can be used suchas multi-spectral values based on filters that are not necessarily basedon a linear transformation of the color matching functions to representcolor.

[0008] A printer model is a mathematical relation that expresses colorvalues as a function of colorants for a given printer. The variables forthe colorants are denoted as C₁, C₂, . . . , C_(n) with n the dimensionof the colorant space. An n-ink process is completely characterized byits colorant gamut with a number of colorant limitations and the printermodel. Because of this close relationship between an n-ink process andthe printer model, the operations typical for a printer model are alsodefined for the n-ink process.

[0009] The printer model is often based on a printer target. Such atarget consists of a number of uniform color patches, defined in thecolorant space of the printing device. The printer target is printed andmeasured, and based on the values of the patches in colorant space andthe measured values, the printer model is made. A printer target isnormally based on a number of sampling points along the differentcolorant axes. Based on the sampling points a regular grid can beconstructed in colorant space of which a number of grid points arecontained by the printer target. Hence a target can be said to becomplete or incomplete. EP-A-1 146 726 describes complete and incompleteprinter targets and is incorporated herein in its entirety for referencefor background information.

[0010] Creating the printer model is also called characterizing theprinter; this is an important step in the consistent reproduction ofimages. Before a printer is characterized, it is first calibrated, i.e.put in a standard state. When the printer model is created, it can beinverted in order to obtain a so-called characterization transformation.The characterization transformation transforms given colors from colorspace (typically CIELAB) to the colorant space of the printing device,whereas the printer model transforms given colorant values in thecolorant space of the printer to color values in color space.

[0011] In characterizing printing devices, in most casesmulti-dimensional Look Up Tables (LUT's) are used. A typical example ofsuch a characterization system is represented by the ICC profile format(ICC stands for International Color Consortium). For printers, both theforward and the inverse relation is needed. The forward relation,embodied in the forward LUT, predicts the color values in function ofgiven colorant values, i.e. it represents the printer model. The inverserelation, embodied in the inverse LUT, gives the colorant valuesrequired to obtain given color values, i.e. it represents thecharacterization transformation of the printer.

[0012] A LUT is often characterized by a number of sampling points peraxis. Based on these sampling points, usually a regular grid isconstructed. However, it is also possible to construct LUT's withnon-regular grids. Also in this case the LUT's can be characterized bysampling points per axis but not all combinations of the sampling pointsof the different axes result in grid points. We refer to patentapplication EP-A-1 146 726 for more information on grids, printermodels, complete and incomplete printer targets, and related terms, andto patent application EP-A-1 083 739 (herein incorporated by referencein its entirety for background information) for more information oncalibration, characterization, and other relevant terms.

[0013] In known systems, the sampling points of a LUT are chosen atpredetermined values, e.g., for sampling points along a colorant axis c,at colorant values c=0%, 25%, 50%, 75% and 100%.

[0014] Several printers have, for one or more of the colorants,multi-density inks, i.e. two or more inks that have a different density.By means of multi-density inks, the apparent visual resolution of theprinted images can be increased. Multi-density inks can be used inseveral ways; however, if the calibration is based on 1-ink processes,the relation between the multi-density inks is fixed. If there is alight and heavy ink for cyan for example, a relation is given thatconverts a global cyan value to a light and a heavy cyan value. Hencethe printer is still considered as a CMYK device, but internally theglobal ink values can be converted to multi-density ink values. Therelation between a global ink value for a particular colorant and themulti-density ink values is given by an ink splitting table, also calledink mixing table.

[0015] There is still a need for an improved method for characterizing aprinting device.

SUMMARY OF THE INVENTION

[0016] The present invention is a system for characterizing a printingdevice as claimed in independent claim 1, and a method therefore asclaimed in independent claim 8. Preferred embodiments of the inventionare set out in the dependent claims.

[0017] The present invention concerns the selection of sampling pointsfor the characterization of a printing device. In a preferred embodimentof the invention, at least one of the sampling points of the forwardlook up table used in the characterization is an ink changing point,which is, in this document, a point where the ink mixing table has anon-smooth behavior.

[0018] An advantage of the invention is that it allows to obtain a muchmore accurate printer model.

[0019] Another advantage is that color changes in the reproduced imagesare represented smoothly.

[0020] Further advantages and embodiments of the present invention willbecome apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is described with reference to the followingdrawings without the intention to limit the invention thereto, and inwhich:

[0022]FIG. 1 shows an ink mixing table;

[0023]FIG. 2 represents a graph of CIE lightness L* for the ink mixingtable of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0024] As mentioned already above, if a printing device hasmulti-density inks for at least one of its colorants, and thecalibration is based on 1-ink processes, customarily an ink splittingtable (or ink mixing table) is used. Such an ink splitting table givesthe relation between a global ink value for the colorant for whichmulti-density inks are used and the multi-density ink values.

[0025] If the multi-density inks include a light and heavy ink, thentypically the light ink increases till a given global value. From thispoint on the light value decreases and the heavy ink increases. At theglobal value for which the light ink reaches its maximum and the heavyink increases, the behavior of the printer might change suddenly. Henceit is important that the behavior of the printer is properly modeled forthis value. Therefore it is desirable that the forward look up tablecontains colorant combinations, preferably all colorant combinations,related to this global value. If for example there are two cyanmulti-density inks and one magenta, one yellow and one black ink, andthe global value at which the light cyan is maximal is 40%, then the 40%value is preferably a sampling point along the global cyan axis.

[0026]FIG. 1 shows an ink mixing table 40 for a light and a heavymulti-density ink of a particular colorant (e.g. cyan). FIG. 1 gives theamount of light ink, curve 21, and the amount of heavy ink, curve 22, asa function of the global colorant value c for the particular colorant,which is indicated along axis 10. In the embodiment of FIG. 1, theamount of light ink 21 reaches a maximum at a global colorant value ofc=40% and then decreases to zero at c=70%. The amount of heavy ink 22 onthe other hand increases from zero, at c=40%, up to a maximum reached atc=100%. The amounts of light and heavy ink are given by the ordinatevalues of curves 21 and 22, i.e. by the values along axis 20. In FIG. 1,the maximum amount for the light ink is 52% and that for the heavy ink73%. As is the case in the embodiment depicted in FIG. 1, the maximumamounts do not have to be 100% but can be lower, in order to reproducecolors with less ink.

[0027] The behavior of the ink mixing table of FIG. 1 is not smooth atc=40% and c=70%. This behavior in most cases induces a nonsmooth changein the color values. In FIG. 2 one of the color values, CIE lightnessL*, is represented for the mixing of the inks represented in FIG. 1.FIG. 2 indeed shows a non-smooth change of lightness L* for c=40% andc=70%, as indicated by the solid curve 23 (remark: in FIG. 2, point c=0%of curve 23 has a high value of L*, e.g. L*=100 if L* is determined withrespect to the so-called ‘white of the paper’, while point c=100% ofcurve 23 has a low value of L*, e.g. L*=40). If the forward look uptable contains the sampling points 16 and 17 at c=40% and c=70%, thecoordinates of the points 28 are known and hence the printer model canpredict this non-smooth behavior. If however the forward look up tabledoes not contain these sampling points 16, 17, but for example only thesampling points 11-15 at respectively c=0%, 25%, 50%, 75% and 100%, onlythe coordinates of the points 27 are known. There is thus no informationavailable of the color behavior at c=40% and 70%, and hence the modelwill not be able to predict the non-smooth effects at c=40% and 70%. TheL* values predicted by a printer model based on sampling points 11-15 isshown in FIG. 2 by the dotted curve 24. The errors made at c=40% and 70%are quite serious; they are indicated by the line segments 31 and 32,which are the differences between the ordinate values of the solid curve23 and the dotted curve 24.

[0028] The effect of properly modeling the printer can easily be shownfor the reproduction of neutral colors, i.e. white, grays and blacks,and this especially for low GCR (Gray Component Replacement) values. LowGCR means that the gray values are made with almost a minimal amount ofblack, i.e., for a CMYK printing device, mainly with CMY. If a minimalamount of black is used for reproducing gray values, the gray values arereproduced in a very unstable way. This means that a small change of thecolorant values has a maximal effect on the required color. If however,a maximal amount of black is used (high GCR), the grays are reproducedin a very stable way. This effect can be easily seen as changes in theCMY values often induce a small color shift of the neutrals and thisshift changes in most cases smoothly from a dark gray to a light gray.As a small color change in the neutrals (saturation mismatch, or a*b*mismatch as represented in CIELAB space) is the most visible for graycolors, it is important to reduce this effect as much as possible.

[0029] As is clear from the explanation above, to model the printerproperly in case of multi-density inks and the use of ink mixing tables,it is advantageous to include the values for which a multi-density inkchanges non-smoothly. The points that correspond to these values arecalled ink changing points in this document. Typical ink changing pointscorrespond to values at which an additional ink starts (in FIG. 1: c=40%for curve 22 of the heavy ink), at which an ink reaches a maximum (c=40%for curve 21 of the light ink), becomes constant, or zero (c=70% forcurve 21 of the light ink). Mathematically speaking, at an ink changingpoint the derivative of the ink mixing table is not continuous and hencethe ink mixing table has a non-smooth behavior at the ink changingpoint.

[0030] The inverse LUT may be obtained by inverting the printer model.In case of multi-density inks, the inverse LUT preferably includes oneor more sampling points at which the multi-density inks have anon-smooth behavior. The inverse LUT has sampling points along the coloraxes. Typically CIELAB is used as color space. Within this space, colorsfor which a* and b* are substantially zero are considered to be neutral.For neutrals, the lightness component L* varies from 0 to 100, wherein 0corresponds to black and 100 to pure white. As the neutral colors arevery difficult to reproduce, it is advantageous to choose a propersampling along the lightness axis. For the same reason as for theforward model, lightness values for which one of the multi-density inksbehaves non-smoothly are preferably sampling points. In our example ofFIGS. 1 and 2 with two multi-density inks, these preferred samplingpoints along the lightness axis correspond to the gray values for whichthe multi-density colorant value is c=40% and c=70%.

[0031] A sampling point belonging to the grid of an inverse LUT forwhich one of the corresponding separation values is an ink changingpoint is considered as an ink changing point of this LUT.

[0032] In a system that includes a look up table for characterizing aprinting device, and in a method for constructing such a look up table,it is thus advantageous that the look up table includes at least one inkchanging point. The look up table may be a forward look up table. Thelook up table may be an inverse look up table.

[0033] The invention was discussed especially with respect to CMYK inkjet printers, but the present invention is not limited to theembodiments discussed above. The invention is also applicable toprinting devices using other printing technologies, such aselectrophotography, thermal transfer, dye sublimation. Other colors thanCMYK may be applied; the printer may have more, or less, than fourcolorants.

[0034] Having described in detail preferred embodiments of the currentinvention, it will now be apparent to those skilled in the art thatnumerous modifications can be made therein without departing from thescope of the invention as defined in the appending claims.

[0035] List of Reference Signs

[0036] 10: colorant axis

[0037] 11-17: sampling point

[0038] 16: ink changing point

[0039] 17: ink changing point

[0040] 20: ink amount

[0041] 21: curve

[0042] 22: curve

[0043] 23: curve

[0044] 24: curve

[0045] 27: point

[0046] 28: point

[0047] 31: error

[0048] 32: error

[0049] 40: ink mixing table

What is claimed, is:
 1. A system for characterizing a printing device,the system comprising a forward look up table for obtaining color valuesas a function of given colorant values for said printing device, saidforward look up table having a plurality of sampling points, whereinsaid plurality of sampling points includes an ink changing point of saidprinting device.
 2. The system according to claim 1, wherein saidprinting device has, for a particular colorant, a light ink and a heavyink and wherein said ink changing point is selected from the group of afirst point where an amount of said light ink reaches a maximum and asecond point where said amount of said light ink becomes zero.
 3. Thesystem according to claim 1 wherein said plurality of sampling pointsincludes all ink changing points of said printing device.
 4. The systemaccording to claim 1 wherein said printing device is an ink-jet printer.5. The system according to claim 2 wherein said printing device is anink-jet printer.
 6. The system according to claim 3 wherein saidprinting device is an ink-jet printer.
 7. The system according to anyone of the preceding claims further comprising said printing device. 8.A method for constructing a look up table for characterizing a printingdevice, wherein said look up table is a forward look up table forobtaining color values as a function of given colorant values for saidprinting device, the method comprising: determining a sampling point ofsaid forward look up table; determining an ink changing point of saidprinting device; adding said ink changing point to said forward look uptable as an additional sampling point.
 9. The method according to claim8 wherein said printing device has, for a particular colorant, a lightink and a heavy ink, the method further comprising: selecting said inkchanging point from the group of a first point where an amount of saidlight ink reaches a maximum and a second point where said amount of saidlight ink becomes zero.
 10. The method according to claim 8 furthercomprising: determining all ink changing points of said printing device;adding all said ink changing points to said forward look up table. 11.The method according to claim 9 further comprising: determining all inkchanging points of said printing device; adding all said ink changingpoints to said forward look up table.